Using a measured error to determine coefficients to provide to an equalizer to use to equalize an input signal

ABSTRACT

Provided are a read channel, storage drive and method using a measured error to determine coefficients to provide to an equalizer to use to equalize an input signal. A read channel is incorporated in a storage device to process signals read from a storage medium. An equalizer uses coefficients to equalize input read signals to produce equalizer output signals. A detector processes adjusted equalizer output signals to determine output values comprising data represented by the input read signals. An equalizer adaptor is enabled to provide a reference measured error and coefficients used to produce the adjusted equalizer signals that are associated with the reference measured error. The equalizer adaptor computes new equalizer coefficients to use to equalize input read signals that result in a new measured error from the detector and computes a new measured error for the new equalizer coefficients. The equalizer adaptor determines whether the new measured error is degraded with respect to the reference measured error and saves the new equalizer coefficients and the new measured error in response to determining that the new measured error is not degraded with respect to the reference measured error. The equalizer adaptor provides the equalizer coefficients associated with the reference measured error to the equalizer to use to equalize input read signals in response to determining that the new measured error is degraded with respect to the reference measured error.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a system and device for using ameasured error to determine coefficients to provide to an equalizer touse to equalize an input signal.

2. Description of the Related Art

Magnetic tape cartridges include magnetic tape to store data to be savedand read back at a subsequent time. A magnetic tape drive writes thedata to magnetic tape, typically as a set of parallel tracks, andsubsequently a magnetic tape drive reads back the data. To read back thedata, a magnetic tape drive typically comprises parallel read heads toread each of the parallel tracks, a drive system for moving a magnetictape with respect to the read heads such that the read heads may detectmagnetic signals on the magnetic tape, and a read channel for digitallysampling magnetic signals sensed by the read heads and providing digitalsamples of the magnetic signals. The digital samples are then decodedinto data bits, and the data bits from the parallel tracks are combinedinto the data that was saved. The read channel typically requires anequalizer for each of the read heads to compensate for the change in thesignal due to the magnetic recording properties of the write head, themagnetic tape, and the read head. Magnetic tape cartridges may beinterchanged between tape drives, such that a magnetic tape written onone tape drive will be read by another tape drive. Variation in theresponse of the read heads to the variously written magnetic tapes mayresult in unacceptably poor read back of the recorded signals.

Adaptive equalizers implemented in magnetic tape drives solve a set ofequations to find the equalizer characteristic that reduces the errorbetween the desired and actual amplitudes. Thus, the equalizer might becomputed at the beginning of use with respect to a magnetic tape, orrecomputed a few times during use. Further, the desired amplitudes maybe difficult to estimate. Hence, in many instances, the desiredamplitudes are best estimated by employing a signal having knowncharacteristics, such as a synchronization signal, or a data setseparator signal, and not the random data signals.

In magnetic tape, the recording characteristics may not only vary fromtrack to track, but may as well vary in a continuous fashion along atrack or tracks. Thus, a selected equalizer characteristic, althoughsatisfactory at the beginning or at some specific track location of amagnetic tape, may lead to an increase in data read errors at some pointalong the track. Further, different tape drives from differentmanufacturers may write data sets to a tape cartridge having differentmagnetic properties, i.e., different signal-to-noise ratios. Moreover,the tape drive is required to read tapes that are manufactured byseveral different vendors, all having slightly different magneticrecording properties. Yet further, data sets may have been written tothe tape cartridge under a wide range of environmental conditions, whichdiffer from the conditions at the time the data is read.

To address the problems mentioned above, a least mean squares (LMS)algorithm can be used to adjust the coefficients of an equalizer thatoperates on the read-back signal at the output of the analog-to-digital(A/D) converter and provides continuous adjustment of the equalizercharacteristic. However, in current implementations, a relatively largenumber of tap coefficients of the equalizer, must be fixed, e.g. as manyas 30-40%, in order to ensure stable operation of the adaptivealgorithm. This limits the ability of the equalizer to fully adapt tochanging conditions. If a tape drive is operating in an environmentwhere data sets can have significantly different magnetic reportingproperties; it is desirable to have most equalizer coefficients adaptingto the read-back signal.

SUMMARY

Provided are a read channel, storage drive and method using a measurederror to determine coefficients to provide to an equalizer to use toequalize an input signal. A read channel is incorporated in a storagedevice to process signals read from a storage medium. An equalizer usescoefficients to equalize input read signals to produce equalizer outputsignals. A detector processes adjusted equalizer output signals todetermine output values comprising data represented by the input readsignals. An equalizer adaptor is enabled to provide a reference measurederror and coefficients used to produce the adjusted equalizer signalsthat are associated with the reference measured error. The equalizeradaptor computes new equalizer coefficients to use to equalize inputread signals that result in a new measured error from the detector andcomputes a new measured error for the new equalizer coefficients. Theequalizer adaptor determines whether the new measured error is degradedwith respect to the reference measured error and saves the new equalizercoefficients and the new measured error in response to determining thatthe new measured error is not degraded with respect to the referencemeasured error. The equalizer adaptor provides the equalizercoefficients associated with the reference measured error to theequalizer to use to equalize input read signals in response todetermining that the new measured error is degraded with respect to thereference measured error.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates an embodiment of a tape drive.

FIG. 2 illustrates embodiments of a read channel in the tape drive.

FIGS. 3 and 4 illustrate an embodiment of operations to determinecoefficients to provide to an equalizer.

DETAILED DESCRIPTION

This invention is described in preferred embodiments in the followingdescription with reference to the Figures, in which like numbersrepresent the same or similar elements. While this invention isdescribed in terms of the best mode for achieving this invention'sobjectives, it will be appreciated by those skilled in the art thatvariations may be accomplished in view of these teachings withoutdeviating from the spirit or scope of the invention.

FIG. 1 illustrates an embodiment of a magnetic tape drive 10. Themagnetic tape drive provides a means for reading and writing informationwith respect to a magnetic tape 14 of a magnetic tape cartridge 12.Magnetic tape cartridges include a magnetic tape storage medium to storedata to be saved and read at a subsequent time. Further, the magnetictape cartridges may be interchanged between tape drives, such that amagnetic tape written on one tape drive will be read by another tapedrive. The magnetic tape cartridge 12 comprises a length of magnetictape 14 wound on one or two reels 15, 16.

A single reel magnetic tape cartridge 12 is illustrated, examples ofwhich are those adhering to the Linear Tape Open (LTO) format. Anexample of a magnetic tape drive 10 is the IBM 3580 Ultrium magnetictape drive based on LTO technology. A further example of a single reelmagnetic tape drive and associated cartridge is the IBM 3592TotalStorage Enterprise magnetic tape drive and associated magnetic tapecartridge. An example of a dual reel cartridge is the IBM 3570 magnetictape cartridge and associated drive. In alternative embodiments,additional tape formats that may be used include Digital Linear Tape(DLT), Digital Audio Tape (DAT), etc.

The magnetic tape drive 10 comprises one or more controllers 18 of arecording system for operating the magnetic tape drive in accordancewith commands received from a host system 20 received at an interface21. A controller typically comprises logic and/or one or moremicroprocessors with a memory 19 for storing information and programinformation for operating the microprocessor(s). The program informationmay be supplied to the memory via the interface 21, by an input to thecontroller 18 such as a floppy or optical disk, or by read from amagnetic tape cartridge, or by any other suitable means. The magnetictape drive 10 may comprise a standalone unit or comprise a part of atape library or other subsystem. The magnetic tape drive 10 may becoupled to the host system 20 directly, through a library, or over anetwork, and employ at interface 21 a Small Computer Systems Interface(SCSI), an optical fiber channel interface, etc. The magnetic tapecartridge 12 may be inserted in the magnetic tape drive 10, and loadedby the magnetic tape drive so that one or more read and/or write heads23 of the recording system read and/or write information in the form ofsignals with respect to the magnetic tape 14 as the tape is movedlongitudinally by motors 25 which rotate the reels 15, 16. The magnetictape typically comprises a plurality of parallel tracks, or groups oftracks. In certain tape formats, such as the LTO format, the tracks arearranged in a serpentine back and forth pattern of separate wraps, as isknown to those of skill in the art. Also as known to those of skill inthe art, the recording system may comprise a wrap control system 27 toelectronically switch to another set of read and/or write heads, and/orto seek and move the read and/or write heads 23 laterally of themagnetic tape, to position the heads at a desired wrap or wraps, and, insome embodiments, to track follow the desired wrap or wraps. The wrapcontrol system may also control the operation of the motors 25 throughmotor drivers 28, both in response to instructions by the controller 18.

Controller 18 also provides the data flow and formatter for data to beread from and written to the magnetic tape, employing a buffer 30 and aread/write channel 32, as is known to those of skill in the art.

The tape drive 10 system further includes motors 25 and reels 15, 16 tomove the magnetic tape 14 with respect to the read head(s) 23 such thatthe read head(s) may detect magnetic signals on the magnetic tape. Aread channel of the read/write channel 32 digitally samples the magneticsignals detected by the read head(s) to provide digital samples of themagnetic signals for further processing.

FIG. 2 illustrates an embodiment of a portion of a read channel of theread/write channel 32 of FIG. 1 including an embodiment of an equalizeradaptor that uses measured errors to adjust the coefficients used by theequalizer. In embodiments where the read channel may concurrently read aplurality of parallel tracks, the read/write channel 32 may comprise aplurality of read channels, in which some of the components may beshared.

FIG. 2 illustrates an embodiment of certain, but not all, of thecomponents of a read channel 50 to provide digital samples of themagnetic signals sensed by the read head 23. An equalizer 52 receives asignal 54 from an analog-to-digital converter (ADC) (not shown), whichconverts analog signals read from tape to digital samples that can beprocessed by the equalizer 52. In one embodiment, the equalizer 52 maycomprise a finite impulse response (FIR) filter having adjustable tapcoefficients. The equalizer 52 processes the digital samples to achievea desired signal characteristic at the equalizer output, thuscompensating for differences in the signal due to the magnetic recordingproperties of the write head, the magnetic tape, and the read head. Theprocessing is based on a series of specific functions employing tapcoefficients, that may be adapted by an equalizer adaptor 56. Theequalized digital samples output by the equalizer 52 are supplied to aninterpolator 58 comprising a timing circuit to provide signal samplesthat are spaced by a bit or symbol interval.

Determination of the information content of the magnetic signalsrequires determining the timing or position of magnetic transitions ofthe magnetic signals. Typically, the sample signals 54 are takenasynchronously with respect to the clock used to write the data on themagnetic tape. The interpolator 58 interpolates the asynchronous samplesinto a set of samples that can be considered to be synchronous with thewrite clock or with the positions of the magnetic recording transitions.A timing control component 60 may include phase-error generation logic,a phase locked loop (PLL) and phase interpolation logic to derive areference for the interpolator 58 to provide the synchronous samples. Avariable gain amplifier circuit (VGA) 62, which may comprise a customdesigned logic circuit, adjusts the gain of the signals from theinterpolator 58 to scale the samples of the gain adjusted synchronoussignal 70 to optimal levels.

A detector 64 receives the gain adjusted synchronous signal 70 from theVGA 62 to determine the data information represented by the digitalsamples. The determined data-information which is represented by astream of detected bits (i.e., zeros and ones) is outputted as signal 65for further processing. In one embodiment the detector 64 may be athreshold device followed by additional logic to determine the datainformation. Besides determining the data information, the detector 64may generate an estimate of the synchronized, gain adjusted equalizeroutput signal using the threshold device and provide this estimate asoutput value 68. In another embodiment the detector 64 may be a sequencedetector. This sequence detector may provide the data information andmay additionally generate an estimate of the synchronized, gain adjustedequalizer output signal using (tentative) decisions from the detectortrellis and provide this estimate as output value 68. Similarly, thesequence detector may generate another estimate of the synchronized,gain adjusted equalizer output signal using (tentative) decisions from adifferent location in the detector trellis and provide this estimate asoutput value 69. The output value 68 from the detector 64, i.e., theestimate of the desired value, and the gain adjusted synchronous signal70 is provided to a gain control 66 that calculates an error signal toadjust the VGA circuit 62. Similarly, the output value 68 from thedetector 64 and the gain adjusted synchronous signal 70 are used by thetiming control 60 to adjust the interpolator 58. Further, the outputvalue 68 from the detector 64 and the gain adjusted synchronous signal70 are provided to a least mean squares (LMS) computation 74 componentof the equalizer adaptor 56 to generate an error signal to adjust thecoefficients used by the equalizer 52. In an alternative embodiment, theLMS computation 74 may use the detector output 68 and the output fromthe interpolator 58 to determine an error used to adjust thecoefficients.

The error signals calculated by the equalizer adaptor 56, the gaincontrol 66, and the timing control 60 are signed values determining theamplitude and the direction of the error. A simplified version of anamplitude independent error signal may only use the sign of the errorsignal to indicate the direction of the error.

In one embodiment, the equalizer 52 may adjust the input signal 54 byusing a finite impulse response (FIR) filter producing output (Z_(n))based on coefficients (c_(i,n)) supplied by the LMS computation 74,adjusted by the error signal comprising the difference of the output ofthe detector 68 (desired value) and the gain adjusted synchronous signal70 or the output from the interpolator 58. Equation (1) describes thegeneration of an equalized signal (Z_(n)) using N samples (x_(n−i)) frominput 54 and N filter coefficients (c_(i,n)). The coefficients (c_(i,n))comprise an index n denoting the time cycle and an index i denoting thecoefficient number,

$\begin{matrix}{Z_{n} = {\sum\limits_{i = 0}^{N - 1}{c_{i,n}{x_{n - i}.}}}} & (1)\end{matrix}$

The equalizer adaptor 56 adjusts coefficients (c_(i,n)) according to theerror signal (e_(n)) calculated from the detector output 68 and the gainadjusted synchronous signal 70 (or the output from the interpolator 58).A programmable parameter (α) controls the speed at which thecoefficients converge, i.e., the larger alpha (α) the faster theconvergence. In one embodiment, the LMS computation 74 calculatesadjusted coefficients (c_(i,n)) by using the LMS algorithm shown belowin equation (2). The adjusted coefficients are then used by theequalizer 52 in equation (1) to calculate the equalized signal (Z_(n)).c _(i,n+1) =c _(i,n) −αe _(n) x _(n−i), where i=0, 1 . . . N−1   (2)

In certain embodiments, because some small amount of interaction existsbetween the equalizer and the timing control loops, the LMS computation74 may need to be constrained in order to avoid possible ill-convergenceproblems. This can be achieved by fixing (i.e., not adjusting) some ofthe equalizer coefficients (c_(i,n)). Equation (3) below shows how thecoefficient (c_(i,n)) may be calculated, such that certain coefficientsare fixed to their current value if they are at an index (i) that is amember of the set of fixed coefficients (I).

$\begin{matrix}{c_{i,{n + 1}} = \left\{ \begin{matrix}{{c_{i,n} - {\alpha\; e_{n}x_{n - i}}},} & {{\mathbb{i}} \notin I} \\{c_{i,n},} & {{\mathbb{i}} \in I}\end{matrix} \right.} & (3)\end{matrix}$

Thus, if the coefficient is a member of the set of fixed coefficients(I), the coefficient for the time cycle (n+1), c_(i, n+1), is set to thecoefficient c_(i, n), from the previous time cycle (n), i.e., the tapcoefficient is fixed. If the coefficient (c_(i,n)) is not a member ofthe set of fixed coefficients, then it is adjusted. The designer of theread channel 50 may determine the number of coefficients to fix based onempirical testing.

The equalizer adaptor 56 further includes a Mean Squared Error (MSE)computation 76 component used to compute an MSE from the error signal 78comprising the difference of the output 69 and the gain adjustedsynchronous signal 70. An MSE control 80 uses the measured MSE value todetermine whether to use the recently computed coefficients from the LMScomputation 74 or to reuse old coefficients computed previously. The MSEmay be computed as an averaged value of the squared value of thedetector error signal 78 (err) according to equation (4) below:mse(n)=α·err ²(n)+(1−α)*mse(n−1).   (4)

The mse(n−1) indicates the MSE value from the previous time cycle, and(err(n)) provides the current detector error signal 78. The variable (α)comprises a weight factor defining a time constant for the erroraveraging operation, and may comprise 1/1024=0.00098. The MSE is arepresentation of the inverse of the signal-to-noise ratio (SNR), suchthat if the MSE decreases, then equalizer adaptation has improved,whereas if the MSE increases, then there has been a degradation ofperformance.

FIGS. 3 and 4 illustrate an embodiment of operations performed by theequalizer adaptor 56 components, including the MSE control 80, todetermine the coefficients that will be provided to the equalizer 52 toadjust the input signal 54. FIG. 3 illustrates an embodiment ofoperations the MSE control 80 performs in an initialization phase todetermine an initial estimate of the MSE which is used as a qualifier toselect the best possible equalizer coefficients to provide to theequalizer 52. As discussed the MSE and SNR reflect the quality of thegain adjusted synchronous signal 70 that is provided to the detector 64.The initialization phase of FIG. 3 may be performed when starting toread data or when reading a new data set from the tape medium todetermine an initial estimate of the measured error (e.g. MSE). Inresponse to beginning the initialization phase (at block 100), an MSEcomputation phase begins (at block 102) to perform the followingoperations. The MSE computation phase may be performed a fixed number ofcycles associated with reading a fixed number of input signals 54. Uponreceiving (at block 104) an input signal 54, the MSE control 80 providesfixed (initial) tap coefficients to the equalizer 52 to use to compute(at block 106) an equalized signal. Upon computing (at block 108) thedetector error signal 78 based on the difference of the output 69 andthe gain adjusted synchronous signal 70 equalized using the fixedcoefficients, the MSE computation 76 calculates (at block 110) ameasured error (MSE) from the detector error signal 78.

If (at block 112) the end of the initial measured error (MSE)computation phase is not yet reached, control proceeds (at block 112)back to block 102. In one embodiment, the end of the initial measurederror (MSE) computation phase can be determined by evaluating a counterwhich counts the number of cycles that have been spent in the initialcomputation phase loop. If the counter reaches a predefined maximumvalue, convergence of the initial MSE computation is assumed and the endof the initial measured error (MSE) computation phase is declared. Ifthe end of the initial MSE computation phase is reached, then controlproceeds to block 114.

If (at block 114) a saved reference measured error (MSEref) is zero,i.e., the computed initial MSE is the first computed MSE, then themeasured error (MSE) provided at block 110 is saved (at block 116) asthe reference measured error (MSEref) and the fixed equalizercoefficients are saved as associated with the reference measured error(MSEref). In one embodiment, if (at block 118) the recently measurederror (MSE) is worse, i.e., greater than, the reference measured error(MSEref) plus some MSE margin (delta) (which means the SNR resultingfrom the equalizer 52 output has degraded beyond the MSE margin), thencontrol proceeds to block 116 to save the recently measured error (MSE)as the reference measured error (MSEref) and the fixed coefficientsassociated with that updated reference measured error (MSEref) in orderto deal with varying SNR conditions in a new data set. If the measurederror (MSE) has not degraded or after saving the recently measured error(MSE) as the reference measured error (MSEref), then control proceeds toblock 150 in FIG. 4 (at block 120) to adjust the coefficients during aruntime mode to provide to the equalizer 52.

FIG. 4 illustrates an embodiment of operations performed by theequalizer adaptor 56 during a runtime mode to provide the equalizer 52with coefficients to use to equalize the input signal 54. At block 150,the LMS algorithm 74 calculates new coefficients based on the gainadjusted synchronous signal 70 (or output signal from the interpolator58) and output value 68 from the detector 64. The equalizer 52 uses (atblock 152) the new equalizer coefficients to adjust the input signal 54to produce an equalized signal for the interpolator 58 to send to thedetector 64. Upon computing (at block 154) the detector error signal 78based on the difference of the output 69 and the gain adjustedsynchronous signal 70, the MSE computation 76 calculates (at block 156)a measured error (MSE) from the detector error signal 78. If (at block158) the measured error (MSE) is worse, i.e., MSE is greater than thereference measured error (MSEref) plus some MSE margin (delta), then theLMS adaptation is stopped and the current equalizer coefficients arefixed and used to equalize the input signal 54 (at block 160). If (atblock 162) the end of a data set has been detected, which may comprise asignal being generated within the tape drive, then control proceeds tothe MSE control 80 which reloads (at block 164) the previously savedequalizer coefficients associated with the current reference measurederror (MSEref) to be used by the equalizer 52. Otherwise, the currentfixed equalizer coefficients are used to equalize the input signal 54(at block 160).

If (at block 158) the measured error signal has not worsened beyond somemargin (delta) and if (at block 166) the end of a data set has beendetected, which may comprise a signal being generated within the tapedrive, then a determination is made (at block 168) whether the measurederror (MSE) at the end of the data set is worse than the referencereferenced measured error (MSEref), i.e., equalizer performance hasdegraded. As discussed, the SNR may be determined to be degraded if thecurrently measured error (MSE) is greater than the reference measurederror (MSEref).

If (at block 168) the measured error (MSE) has not degraded, then theMSE control 80 saves (at block 170) the measured error (MSE) as the newreference measured error (MSEref) and saves the new equalizercoefficients as associated with the new reference measured error(MSEref).

If (at block 168) the measured error (MSE) has degraded, i.e., themeasured error (MSE) is greater than the reference measured error(MSEref), then the MSE control 80 reloads (at block 164) the previouslysaved equalizer coefficients associated with the current referencemeasured error (MSEref) to be used by the equalizer 52. In this way, thecoefficients with the best possible measured error (MSE) are used.

If (at blocks 170 or 164) the equalizer coefficients have been saved orprevious coefficients have been reloaded, then a determination is made(at block 172) as to whether the initialization phase is to be performedat the beginning of a next data set to use fixed coefficients tore-initialize the measured error (MSE) value, or whether the runtimemode continues to have the LMS computation 74 calculate new coefficientsto use without re-initializing the measured error (MSE) value. In oneembodiment, the administrator may set a variable indicating whether tocontinue with the runtime mode or proceed back to the initializationphase to recalculate the MSE to use when beginning to process a nextdata set. For instance, if the administrator determines that it islikely that data sets are written by different tape drives or underdifferent environmental conditions, then the administrator may select todo initialization at the start of each data set to re-initialize thereference MSE (MSEref) based on the anticipated new conditions. In thiscase, control proceeds to block 102 in FIG. 3 to perform an initial MSEcomputation at the beginning of the next data set. Alternatively, if theadministrator determines that the same tape drive is likely used andenvironmental conditions remain relatively unchanged, then it might beadvantageous not to recalculate the reference measured error (MSEref).In this case, control proceeds to runtime mode (at block 150) directly.

Described embodiments provide techniques to determine whether to usepreviously calculated equalizer coefficients or newly calculatedequalizer coefficients to equalize read input signals based on whetherthe measured error (MSE) of the gain adjusted synchronous signal 70 forthe newly calculated coefficients results or does not result inperformance degradation.

The described components of the read channel 50 and equalizer-adaptor 56comprise discrete logic, ASIC (application specific integrated circuit),FPGA (field programmable gate array), custom processors, etc.

The described components of the read channel 50 and the operations ofthe equalizer adaptor 56 described with respect to FIGS. 3 and 4 mayalso be implemented in subroutines in programs or other softwareimplementations executed by a processor. Such programs implementing theoperations of the equalizer adaptor circuit 56, such as those operationsshown in FIGS. 3 and 4, may be implemented in a computer readablemedium, such as magnetic storage medium (e.g., hard disk drives, floppydisks, tape, etc.), optical storage (CD-ROMs, DVDs, optical disks,etc.), volatile and non-volatile memory devices (e.g., EEPROMs, ROMs,PROMs, RAMs, DRAMs, SRAMs, Flash Memory, firmware, programmable logic,etc.), etc. The code implementing the described operations may furtherbe implemented in hardware logic (e.g., an integrated circuit chip,Programmable Gate Array (PGA), Application Specific Integrated Circuit(ASIC), etc.).

Components in FIGS. 1 and 2 shown as separate components may beimplemented in a single circuit device or functions of one illustratedcomponent may be implemented in separate circuit devices.

Those of skill in the art will understand that changes may be made withrespect to the components illustrated herein. Further, those of skill inthe art will understand that differing specific component arrangementsmay be employed than those illustrated herein.

The illustrated operations of FIGS. 3 and 4 show certain eventsoccurring in a certain order. In alternative embodiments, certainoperations may be performed in a different order, modified or removed.Moreover, steps may be added to the above described logic and stillconform to the described embodiments. Further, operations describedherein may occur sequentially or certain operations may be processed inparallel. Yet further, operations may be performed by a singleprocessing unit or by distributed processing units.

The foregoing description of various embodiments of the invention hasbeen presented for the purposes of illustration and description. It isnot intended to be exhaustive or to limit the invention to the preciseform disclosed. Many modifications and variations are possible in lightof the above teaching. It is intended that the scope of the invention belimited not by this detailed description, but rather by the claimsappended hereto. The above specification, examples and data provide acomplete description of the manufacture and use of the composition ofthe invention. Since many embodiments of the invention can be madewithout departing from the spirit and scope of the invention, theinvention resides in the claims hereinafter appended.

1. A method for processing signals read from a storage medium,comprising: using coefficients to equalize input read signals to produceequalizer output signals; processing adjusted equalizer output signalsto determine output values comprising data represented by the input readsignals; providing a reference measured error and coefficients used toproduce the adjusted equalizer signals that are associated with thereference measured error; computing new equalizer coefficients to use toequalize input read signals that result in a new measured error from adetector; computing a new measured error for the new equalizercoefficients; determining whether the new measured error is degradedwith respect to the reference measured error; saving the new equalizercoefficients and the new measured error in response to determining thatthe new measured error is not degraded with respect to the referencemeasured error; providing coefficients associated with the referencemeasured error to an equalizer to use to equalize input read signals inresponse to determining that the new measured error is degraded withrespect to the reference measured error; and saving the new equalizercoefficients and the new measured error as the reference measured errorin response to detecting an end of a data set and to detecting animprovement of the new measured error over the reference measured error.2. The method of claim 1, wherein the measured errors are based onadjusted equalizer output signals and output values from the detector.3. The method of claim 1, wherein the measured errors comprise meansquared values of errors measured as a difference of an input to thedetector and an output from the detector, and wherein the measurederrors correspond to a signal-to-noise ratio at the input to thedetector.
 4. The method of claim 1, further comprising: providing fixedcoefficients to the equalizer to use to equalize the read input signalsfor an initial measured error computation phase; and computing themeasured error from the equalized input signal.
 5. The method of claim1, wherein the measured error is based on a difference of an input andoutput of the detector, and wherein the new coefficients are calculatedusing the output of the detector.
 6. A method for processing signalsread from a storage medium, comprising: using coefficients to equalizeinput read signals to produce equalizer output signals; processingadjusted equalizer output signals to determine output values comprisingdata represented by the input read signals; providing a referencemeasured error and coefficients used to produce the adjusted equalizersignals that are associated with the reference measured error; computingnew equalizer coefficients to use to equalize input read signals thatresult in a new measured error from a detector; computing a new measurederror for the new equalizer coefficients; determining whether the newmeasured error is degraded with respect to the reference measured error;saving the new equalizer coefficients and the new measured error inresponse to determining that the new measured error is not degraded withrespect to the reference measured error; providing coefficientsassociated with the reference measured error to an equalizer to use toequalize input read signals in response to determining that the newmeasured error is degraded with respect to the reference measured error;and stopping equalizer adaptation and providing fixed equalizercoefficients to the equalizer to use to equalize the input read signalsuntil an end of a data set is detected in response to determining thatthe new measured error is degraded with respect to the referencemeasured error.
 7. A method for processing signals read from a storagemedium, comprising: using coefficients to equalize input read signals toproduce equalizer output signals; processing adjusted equalizer outputsignals to determine output values comprising data represented by theinput read signals; providing a reference measured error andcoefficients used to produce the adjusted equalizer signals that areassociated with the reference measured error; computing new equalizercoefficients to use to equalize input read signals that result in a newmeasured error from a detector; computing a new measured error for thenew equalizer coefficients based on the input signals equalized usingfixed coefficients; determining whether the new measured error isdegraded with respect to the reference measured error; saving the newequalizer coefficients and the new measured error in response todetermining that the new measured error is not degraded with respect tothe reference measured error; providing coefficients associated with thereference measured error to an equalizer to use to equalize input readsignals in response to determining that the new measured error isdegraded with respect to the reference measured error; saving the newmeasured error as the reference measured error and saving the fixedcoefficients to use to equalize the input signal in response todetermining that there exists no value for the reference measured error;determining whether the new measured error is degraded with respect tothe reference measured error in response to using the fixed coefficientsfor an initial measured error computation phase; and saving the newmeasured error as the reference measure error and saving the fixedcoefficients to use to equalize the input signal in response todetermining that the new measured error is degraded beyond some margin.8. A method for processing signals read from a storage medium,comprising: using coefficients to equalize input read signals to produceequalizer output signals; processing adjusted equalizer output signalsto determine output values comprising data represented by the input readsignals; providing a reference measured error and coefficients used toproduce the adjusted equalizer signals that are associated with thereference measured error; computing new equalizer coefficients to use toequalize input read signals that result in a new measured error from adetector; computing a new measured error for the new equalizercoefficients; determining whether the new measured error is degradedwith respect to the reference measured error; saving the new equalizercoefficients and the new measured error in response to determining thatthe new measured error is not degraded with respect to the referencemeasured error, wherein the operations of calculating the newcoefficients, using the new coefficients to equalize input read signalsto produce equalizer output signals, and determining whether the newmeasured error is degraded are performed until detecting an end of adata set; providing coefficients associated with the reference measurederror to an equalizer to use to equalize input read signals in responseto determining that the new measured error is degraded with respect tothe reference measured error; and determining whether to stop equalizeradaptation and to provide fixed coefficients to the equalizer to use toequalize the input signal in response to determining that the newmeasured error is degraded with respect to the reference measured errorbeyond some margin.
 9. The method of claim 8, further comprising:determining whether the new measured error is degraded; saving the newcoefficients and the new measured error as reference measured error whenthe measured error is not degraded with respect to the referencemeasured error; and reloading the previous coefficients and the previousmeasured error as reference measured error when the measured error isdegraded with respect to the reference measured error.
 10. The method ofclaim 9, wherein a user settable parameter indicates whether to use thefixed coefficients for an initial measured error computation phase toprocess input signals from the next data set.